Abstract/Summary

The lesser sandeel Ammodytes marinus is a critically important mid-trophic species
in the North Sea ecosystem. Seabirds suffered widespread breeding failures in the North Sea in
2004, due to shortages of sandeels, their principal food. Industrial sandeel fisheries also failed in
2003 to 2005. Explaining why sandeels were in short supply is thus critical to understanding and
managing the North Sea ecosystem. Sandeel abundance may be controlled ‘bottom-up’ by food
availability or ‘top-down’ by predation, including fisheries. The relative importance of these 2
mechanisms may vary over space and time, and failure to take account of such variation may lead
to inappropriate management. We summarise the available evidence for top-down or bottom-up
control of sandeel abundance in 2 well-studied North Sea regions differing in many biotic and
abiotic characteristics. In Shetland, recent low abundance of sandeels coincided with record-high
abundance of herring, which may have exerted a top-down predation pressure on sandeels. Off
SE Scotland, where adult herrings are scarce, abundance of sandeel larvae was positively correlated
with plankton abundance, indicating bottom-up control. Seabird breeding failures in this
area in 2004 were linked to extremely low energy content of sandeels. Large-scale sandeel fisheries
have not been operating in either area since 2000. Control of food web structure and function
in the North Sea is thus likely to be complex, with pronounced regional variation. Improved
cooperation between diverse research organisations will be needed to understand this complexity,
and future ecosystem-based management of marine bioresources will need to take the results of
such research into account.